I have had this happen once. I gathered the family to make a Home Depot run. Wife locked and "closed" the door behind her while I was putting my son in the car. We were gone about an hour. When we came back I looked at the back door and it was opened a little. We enter and leave through the back of the house. I had my wife take our son in to the detached garage. I walked up to the house and took my pistol from its holster. As I got closer to the door I could fell the adrenalin start pumping. I slowly open the screen and pushed the door open. I cleared the house and found nothing. Nothing was gone and no one in the house. It was not a good felling to come home to that. All I can guess is that the door was not shut all the way so now the deadbolts are always used to insure the door is closed properly.

And x2 on the 2x4 make it tight though. If those doors are opened a little you can lift them off there tracks.